the impression of green, when received by the eye and interpreted by the human brain; then the light which comes through will be preponderatingly red, and the glass will be called red glass; not because its properties have any relation to red, but because its properties are related to the light complementary to red, and because it therefore ignores the red; that is to say, it ignores the waves which produce a red sensation, and allows them to pass through unchecked.

On the other hand, if the glass is of such a chemical nature that it absorbs and quenches, and, so to speak, utilises, waves which would produce the sensation of blue, then it is called yellow glass; because that is the light which it ignores, and which therefore is able to get through it, or, if it is at all turbid, to be scattered or deflected from it, in which case it is not so much transparent as translucent. Ordinary clear glass can be made translucent by grinding its surface. A good deal of the light is then deflected or scattered irregularly, while the rest gets through, without selective absorption in the visible part of the spectrum. But if the glass is coloured, whether it be transparent or translucent, it selects and absorbs some, and scatters or transmits the rest.

The important thing is that the transparent material adds nothing to the light. It does not act by addition. It acts by subtraction: and the colour that we see is the residue, the part which is not stopped. That is how all pigments act, and accordingly a green pigment is that which absorbs the red. A blue pigment is that which absorbs the yellow. A yellow pigment is that which absorbs the blue. And if we examine with a prism the light transmitted or reflected from a pigment, we can see by the gaps in the spectrum what it has absorbed; and the part of the light that we receive is the difference, or residue. Consequently if we mix several pigments together we shall add their destructive powers, and get nothing but a brown or blackish mud, the different parts of the light being quenched by the different ingredients which have been mixed together. So, as all water-colour artists know by experience, if you want bright, vivid colours, you must not mix the pigments together, but use them singly wherever you want to produce a luminous effect; though the luminosity is always less, and never more, than that of the light itself. There

is always some selective absorption essentially necessary for the production of any colour by means of pigment.

Children and people generally would naturally assume that when light went through, say, a red liquid, it picked up colour from the liquid, and had acquired something which it did not previously possess. That is false. It has picked up or acquired nothing. It has left something behind. It is less than white light, not more. If we were to pass a thread or ribbon through the liquid the conditions would be quite different. A white ribbon passed through a red liquid does acquire colour from the liquid. It is dyed. It has absorbed some of the material; and if delicately weighed, after drying, would be found to weigh more than before. The act of dyeing is the addition of colour. Light which has passed through a coloured substance appears dyed, but it is not. And if you pass a beam of light through a succession of different coloured windows there will ultimately be none of it left. They do not add, they subtract. A medium may thus act in two distinct ways. It may act by addition, or it may act by subtraction. A solid rod or thread, passing through it, may select certain portions of its substance, that is to say, of the deposit which it finds there, and pass on sophisticated by addition which it has picked up from the medium. A light beam or succession of ether waves passing through a medium may find that medium obstructive or destructive to a portion of what it seeks to transmit, and the part that gets through is only a fraction of what entered: it is sophisticated by subtraction. Or both these things may occur at once. The original agency may have something added to it and something subtracted from it, so that in the end what emerges is quite different from what entered. And if the medium is turbid, too, the light may be so scattered and dispersed that nothing is recognisable; or, in some cases, so that nothing may emerge at all. The medium ceases to be a medium, and becomes a mere absorbent material, a black body.

All which things are an allegory. And the interpretation is sufficiently indicated by the word "Medium."

OLIVER LODGE.

BACKWATERS OF BERKSHIRE

BY ROBERT P. TRISTRAM COFFIN

HISTORICALLY, Berkshire as a county saw its best days a thousand years ago, in Alfred's time. The Danes, pillaging the Thames valley from the sea to Oxford, had passed its chalky grandeur by. But one fine red morning they marched westward. All the livelong night the Saxons had waited in their battleburnies hard by the twisted ashtree of Ashdown. Their spears were cold with the dew of the morning, but their hearts like the East, when the Danes came over the shoulder of the downs with the sun. And the shock of their fury gave the redhaired sea hounds sleep under the treeless turf till Doomsday. From any corner of Berkshire you have only to lift your eyes to see shining the banner of Alfred, the great White Dragon of the West Saxons, carved in the highest chalk of the downs to mind men forever of the glory of Wedmore. Popularly he is the White Horse; but he is the most dragonish of dragons that ever guarded treasure hoards of jewels and flagons and got themselves slain for being the first Northern connoisseurs of objets d'art by some yellowbearded prototype of Saint George. At any rate, there he ramps, his coat scraped anew from century to century by descendants of the men of Ashdown, the great seal of Berkshire's day of glory.

Once more the lonely, lovely county of sheep passes for a moment over the threshold of history. Some century later, in the brief heyday of the Danes, Canute sat throned in Faringdon. But of jovial Canute, whose vainglory God's tides alone could humble, as the old legend runs, there remains less trace than of Alfred. Only a great silver drinking horn inscribed with his name keeps his fame in the shadows of an obscure manor house. For the rest, there is some running to and fro of men of arms in Cromwell's time and before-though all the breaking of church glass is always attributed to Cromwell by the countyfolk, since he is the nearest in time-the rest is oblivion. Squires a little more

thickheaded than in other shires, yeomen leaner by virtue of the scanty loam between bracken and downland, many succeeding hosts of poor shepherds living their little days with the music of their beasts in their ears and sun and rain in their faces and God's simplicity in their hearts, passing on through the centuries as peacefully and unnoticed by the world as the stars that march over the downs. So the tale of Berkshire runs.

In this county the world of steam and electricity seems still far away. The towns would be no more than sleepy villages in America. The only industry is the very ancient one to which Odysseus and the other kings came home from Troy, the trade Abraham followed and the other men who discovered the Great Shepherd who is God out in loneliness under the great Arabian stars; shepherds watch the miracle of green grass passing over into the fleece that keeps men warm. The fame of the Berkshire sheep has gone around the earth from this home of the most picturesque and the oldest of men's callings; for, like New England that produces from hungry, sour soil the tastiest of apples, Berkshire out of thin herbage on dry chalk hills has brought forth the finest wool. It is the miracle of the wellspring in the desert, the mercy called Providence.

Though Berkshire lies in a backwater of history, it has one glory that is ever new. There are the downs, great rolling hills patient as time, eloquent in their bareness, majestic in the silence of wide spaces. Other Southern counties have them, but they have been coming up all the way from the Channel to be at their grandest and loneliest here. Berkshire has no need of the sea which makes the magic of many shires. There is about the sweeping downs the eternity of the sea, the calmness of the sea, and, better than on the sea itself, an isolation and solitude that no sails ever break. Here and there are dark patches, forests cohorted in hollows out of the reach of winds. Hidden away in the downs lie villages you may stumble on for an afternoon of as keen delight as any of the great men of great beards ever had in their seventeenth century when new worlds were making the new cosmographers the busiest of mankind. But chiefly upon the downs there are the eternal shadows of clouds marching over the shoulders of the world, a vast silence, a high sense of the en

durance of things, serenity akin to the massing clouds, and a wistful spaciousness. High, windy birds and clouds surely achieve their majestic effects as fully here as over wide waters. And that nothing may be lacking, the pathetic bleatings of sheep far away give the monotone of the sound of the sea. Perhaps it is only natural, then, that so many of the many windburnt lads the Berkshire shepherd finds time to rear, when they come to manhood and ways that must turn away from Berkshire of the scanty livelihood, turn to the richer shire of the sea.

Because of its loneliness and aloofness from the small ways of men, the downland has the aura of legend that such places gather, even in the face of sunshine and wind. Over its slopes Druids have wandered for their mistletoe and holly, stuff of black magic; Roman legionaries surely have been here, for there is the monument of their orderly genius in empire building, the trim fosses and rightangled company streets of one of their camps, perched on the crest of the downs; Saxon and Dane shocked here, and brave bones bleach in the chalk. So it is to be expected that over the downs of a night goes Wayland the Blacksmith, half a jest and yet half a belief when winds are out trampling the midnight downs. This mysterious Saxon god lingered after Thor had faded into his own thunder and maniacal Tiu and golden-haired Freya were gone forever from memory. So useful a deity would escape the Götterdämmerung, would come by night to befriend fair-haired children of those who loved him once, shoeing their horses for a bit of money left on the moors. Not a god of beauty or music, but one who shoes horses well. So homely the first and the last of the pagan deities of our race. There is a mysterious stone, too, with holes where one can blow an eerie trumpeting. The strangest beliefs about it seem gospel truths when plaintive curlews are weaving runes on twilit skies.

But there is more than legend and loneliness in Berkshire. There are villages that artists would love and philosophers choose to dwell in. They would be the old Greek sort of philosophers, men seeking the unities of the universe in fire or water, in laughter or tears. Such men would find their first principles inside whitewashed walls, under thatch with red wallflowers and mosses. Pythagoras in Berkshire would have chosen window